1. Field of the Invention
The present invention relates to a photodiode that is used as a light-sensitive device, and more particularly to a photodiode that is less affected by electromagnetic noise, and also to an optical transmitter-receiver employing such a photodiode.
2. Description of the Prior Art
Photodiodes, which receive light and convert it into an electric signal, are widely used as light-sensitive devices in remote-control, optical-communications, and other equipment. Conventionally, as shown in FIG. 7, a photodiode is produced by forming an n-type layer 52 on a p-type substrate 51, with the bottom surface of the p-type substrate 51 die-bonded with silver paste 53 or the like to a frame 54 that is connected to the ground level. Accordingly, this photodiode has its anode 56 on the frame side, and has its cathode 55 on the chip-surface side where it receives light.
The light incident on the chip surface is then, at the junction surface between the n-type layer 52 and the p-type substrate 51, subjected to photoelectric conversion, and the resulting electric charge appears at the cathode 55 as a signal that indicates the amount of light received. As shown in FIG. 8, the cathode 55 is connected to a signal processing circuit 9 so that the feeble signal obtained from the cathode is first amplified and then subjected to processing that varies from application to application.
Inconveniently, however, in the above photodiode, the cathode realized as the chip surface has a high impedance, and is therefore susceptible to electromagnetic noise that is emitted from a picture tube, computer, or other component included in the equipment in which the photodiode is employed. As a result, the photodiode tends to output false signals. A false signal causes malfunction of the signal processing circuit, and thereby leads to serious problems such as failure in remote control and corrupt information in optical communication.
To avoid such problems, it has been customary to enclose a photodiode in a shield case made of metal so that the shield case shuts out electromagnetic noise and thereby prevents the photodiode from generating false signals. However, enclosing a photodiode in a shield case not only makes the photodiode larger, but also increases the number of components required and thus increases manufacturing cost as well as manufacturing steps. In addition, even though the use of a shield case is an effective way to shut out electromagnetic noise, it does not provide a fundamental solution to the annoying susceptibility of a photodiode to electromagnetic noise.